Micro-electromechanical systems (MEMS) pressure sensors rely on an accurate measurement of the deflection of a suspended membrane (e.g. silicon or silicon nitride). Typically such sensors have well-known piezo resistive or capacitive read-outs. See, e.g., U.S. Pat. No. 8,256,298 to Suijlen et al., “MEMS pressure sensor.” In order for these sensors to have an accurate reference pressure, the cavity underneath the membrane must be sealed perfectly from the environment, which poses strict constraints on the packaging used to seal the cavity. Conventional arrangements employ an expensive dual wafer bonding technique to create a hermetically sealed cavity.
Pressure sensors with a capacitive read-out have clear advantages over pressure sensors with conventional piezo resistive read-out, including very low power consumption and higher sensitivity. For example, certain pressure sensors employ a thin suspended silicon nitride (SiN) membrane as a capacitive MEMS pressure gauge. To achieve the necessary pressure reference, the cavity underneath the SiN membrane is hermitically closed using a plasma-enhanced chemical vapor deposition (PECVD) SiN sealing process. Device performance is largely determined by the physical, mechanical and structural properties of this film and the thickness necessary to prevent holes forming in the membrane. Among other performance factors, film density and composition determine out-gassing behavior and diffusion barrier properties. Internal stress and hence the sensitivity of the pressure sensor are determined by the membrane's thickness and rigidity, which are related.
Because a getter film is currently used in the art to cover the membrane, the getter material is activated once when the membrane is sealed by placing the whole structure in a heated environment.